Field of the Invention
The present invention relates to a method for transferring a mark pattern to a substrate, and to a method for calibrating a metrology tool. The invention further relates to a lithographic apparatus.
Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Complex devices, e.g. integrated circuits, can be manufactured by arranging multiple patterned layers on top of each other, wherein each pattern is transferred to a corresponding layer with the aid of the lithographic apparatus. Although usually the patterns transferred to successive layers are different, it is important for the proper functioning of the devices to accurately position said patterns with respect to each other. Accurately positioning of said patterns with respect to each other can be done by determining the position of a previous pattern, so that a subsequent pattern can be accurately transferred based on the determined position. How well respective patterns are positioned with respect to each other is called the overlay performance.
The position of a previous pattern can be determined by measuring the position of marks which are distributed, usually evenly distributed, over the surface of the substrate. Typically, determination of the position of marks to determine the position of a previous pattern is done in the lithographic apparatus prior to exposure. The lithographic apparatus may therefore comprise a device configured to determine the position of marks, e.g. relative to a substrate holder or metrology frame.
The overlay performance can be measured by comparing the position of marks in one layer with the position of corresponding marks in another layer. The marks used to measure the overlay performance are usually not the same marks as the marks that are used for measuring the position of a previous pattern. Typically, a metrology system separate from the lithographic apparatus is used to measure the overlay performance.
In order to accurately measure the position of marks on a substrate, e.g. to determine the position of a previous pattern or to determine the overlay performance, the lithographic apparatus and/or metrology system need to be calibrated regularly, for instance after a start-up or maintenance operation, but also in order to compensate for any drift occurring over time. Calibration is preferably performed using substrates with marks provided at predefined positions.
The accuracy of the calibration is amongst others determined by the accuracy with which the marks are transferred to the predefined positions on the substrate. It has been found that the accuracy with which the marks are transferred to the substrate is limited due to random errors, such as random errors in the position of parts relative to other parts, random temperature variations, random pressure variations, etc., present in the lithographic apparatus. The random errors may manifest itself during exposure, positioning of the substrate on a substrate holder, aligning the substrate with a patterning device, etc., thereby resulting in a limited accuracy with which the marks are transferred to the substrate.
To reduce the influence of the random errors on the calibration process of the lithographic apparatus and/or the metrology system, multiple substrates may be used and measured by the lithographic apparatus or metrology system to average the results and thus average out the random errors. However, measuring multiple substrates requires a lot of time and may thus have a negative impact on throughput.